Polycrystalline silicon is used as a raw material for single-crystalline silicon substrates for production of semiconductor devices or substrates for production of solar cells. A Siemens method is known as a method for producing polycrystalline silicon. The Siemens method is a method for obtaining a silicon rod by bringing raw material gas containing chlorosilane into contact with a heated silicon core wire, and thereby performing vapor-phase growth of polycrystalline silicon by a CVD method on the surface of the silicon core wire.
When the vapor-phase growth of polycrystalline silicon is performed by the Siemens method, two vertical silicon core wires and one horizontal silicon core wire are assembled in a square arch shape within a reactor of a vapor-phase growth apparatus. Opposite ends of the square arch-shaped silicon core wires are fixed to a pair of metal electrodes arranged on a base plate of the reactor via a pair of core wire holders. A supply port for raw material gas that causes a reaction, and an exhaust port for reactive exhaust gas are also arranged on the base plate. The configuration as described above is disclosed in, for example, Japanese Patent Laid-Open No. 2006-206387 (Patent Literature 1).
Generally, several tens of square arch-shaped silicon core wires respectively fixed to pairs of metal electrodes arranged on a base plate are arranged in a multi-ring fashion within a reactor. In recent years, along with an increase in the demand of polycrystalline silicon, reactors have increased in size so as to increase the amount of production, and a method for depositing a large amount of polycrystalline silicon in one batch has been employed. The number of silicon core wires arranged within each reactor has also increased with the tendency.
However, when the number of silicon core wires placed in a reactor increases, it becomes difficult to stably supply raw material gas to the surface of each silicon rod. When the raw material gas is unstably supplied, the surface of the silicon rod becomes uneven (popcorn), and as a result, the silicon rod has an uneven thickness to cause a shape defect. When the surface of the silicon rod becomes uneven, polycrystalline silicon also tends to grow abnormally. Furthermore, the cleaning effect of pre-shipment cleaning of polycrystalline silicon is significantly reduced. To eliminate the unevenness in the surface of the silicon rod, the temperature (a reaction temperature) of the surface of the silicon rod may be reduced to generate a mild deposition reaction. In this case, however, polycrystalline silicon is deposited at a lower deposition rate, thereby seriously reducing the productivity and the energy efficiency.
Based on such circumstances, various methods for effectively supplying raw material gas to the surface of each silicon rod have been proposed. For example, in methods disclosed in Japanese Patent Laid-Open No. 2010-155782 (Patent Literature 2) and No. 2002-241120 (Patent Literature 3), the positions of a raw material gas supply nozzle and an exhaust port for reactive exhaust gas are variously improved to allow a deposition reaction to proceed effectively.
However, all of the conventional methods relate to an aspect in which raw material gas supplied into a reactor from a raw material supply nozzle is discharged from an exhaust port for reactive exhaust gas in near-one pass. When the reactor has a large size, the supply amount of raw material gas inevitably increases, so that the production cost becomes higher.